1. Field of Invention
The present invention relates to a high dynamic range imager circuit and a method for reading a high dynamic range image; particularly, it relates to such circuit and method wherein a conversion gain for reading an image is adaptively adjusted by adjusting a variable capacitor.
2. Description of Related Art
FIG. 1 shows a prior art high dynamic range image sensor device, which is disclosed by U.S. Pat. No. 7,075,049. As shown in FIG. 1, the high dynamic range image sensor device includes a dual conversion gain transistor (DCGT) and a capacitor CA for switching the image reading conversion gain, in additional to the four transistors in a conventional 4T image sensor device . FIGS. 2A-2B and FIGS. 2C-2D are schematic diagrams showing charge levels at critical nodes of the high dynamic range image sensor device shown in FIG. 1 in low light condition and high light condition respectively, to illustrate the transfer of the photo-generated charges during operation of the high dynamic range image sensor device.
In the low light condition as shown in FIGS. 2A-2B, less charges are generated by the photosensor unit pd (than in the high light condition shown in FIG. 2C). By turning OFF the transistor DCGT, the overall capacitance is lower (the capacitance of the floating diffusion node fd is constant), and therefore as indicated by the level of the floating diffusion charges shown in FIG. 2B, the conversion gain is relatively higher. On the other hand, in the high light condition as shown in FIGS. 2C-2D, more charges are generated by the pd, and because the overall capacitance is higher by turning ON the DCGT, the conversion gain is relatively lower, as indicated by the level of the floating diffusion charges shown in FIG. 2D. By reading and processing the floating diffusion charges of the above two conditions, a high dynamic range image may be obtained.
One of the drawbacks of the prior art is that only two options of the conversion gains are available, i.e., a high conversion gain and a low conversion gain, which are both fixed.
U.S. Pat. No. 7,518,143 discloses a high dynamic range image sensor device with an adjustable conversion gain, but it requires applying three voltage levels to a transfer transistor TG in the patent, wherein a second voltage level is generated by accumulating photo-generated charges.
In view of the foregoing, the present invention provides a high dynamic range imager circuit and a method for reading a high dynamic range image wherein a conversion gain for reading an image is adaptively adjusted by adjusting a variable capacitor.